One of Amazon's biggest potential weapons as it takes on Apple's market-dominating iTunes music store: a huge cadre of affiliates which it pays commissions to refer sales. In an email sent to affiliates this morning, Amazon said it would pay a 20% retail commission for MP3 sales through Dec. 31, and a 10% rate thereafter. Apple, meanwhile, offers a 5% commission -- which we don't see changing anytime soon. By linking visitors to Amazon's store, affiliates make about 18 cents per 89-cent download referral, while they would make about 5 cents for referring a sale to iTunes.